The present invention relates to methods of tension leveling strip steel and, more particularly, methods of tension leveling strip steel to produce substantially flat strips of sheet steel.
In the process of rolling metal into strips, certain variables exist which create a lack of flatness that causes the strip of material to deviate from a planar configuration. In order to correct the defects which cause a variation in uniformity across the width of the rolled strip, a process known as tension leveling is employed.
A tension leveling apparatus typically includes sets of upper and lower work rolls which are nested relative to each other and are supported by back-up rolls along their lengths. Metal strip is fed through the nested rolls in a serpentine path and at least the portion of the strip passing through the leveling apparatus is placed under tension and elongated. As the strip passes through the work rolls, it is bent around the radii of the work rolls so that the upper and lower surfaces of the strip are alternately elongated and compressed in a tranverse direction. The result is an elongation of the strip across its width which imparts the requisite flatness to the strip. An example of such a tension leveling apparatus is disclosed in Bradlee U.S. Pat. No. 4,635,458, the disclosure of which is incorporated herein by reference.
However, the effectiveness of such tension leveling devices in producing uniformity across the width of metal is reduced as a result of variations in yield strength across the width of the strip. Such variations result from the casting technique and are more pronounced in the older ingot casting processes than in the more recently developed continuous casting processes. Because the yield stress varies across the sheet, the elastic strain imparted by a tension leveling apparatus also varies. If the sheet edge has a lower yield strength, as is frequently the case, the center portion of the sheet will contract more than the edges after the strip emerges from the tension leveling apparatus.
This additional contraction of the center area causes the metal in the edges of the strip to be placed in compression, thereby causing the edges to collapse to form a wavy edge. Accordingly, there is a need for a technique for compensating for variations in yield strength across the width of a sheet which is to be tension leveled in the aforementioned manner.